Conventionally, optical pickup devices are known that are used to write and read information optically to and from an optical disk to and from which information is optically written and read, such as magneto-optical disk, phase-change optical disk or the like.
As a typical one of the optical pickup devices of the above type, below a conventional optical pickup device including an optical system will be described below, generally indicated with a reference 201 and configured as in FIG. 1. As shown in FIG. 1, the optical system 201 includes, in the order following the light path, a light source 211 that emits a laser beam for irradiation to an optical disk 204 set in place, a composite optical element 212 including a three-beam generating diffraction grating 212a that splits, by diffraction, the light emitted from the light source 211 into three beams and a beam-splitter diffraction grating 212b that separates the outgoing light and return light from the optical disk 204 from each other, a limiting aperture 214 that limits the outgoing light to a predetermined numerical aperture NA, an objective lens 215 that focuses the outgoing light onto the optical disk 204, and a photodetector assembly 216 that detects the return light from the optical disk 204. It should be noted here that the “laser beam emitted from the light source” or “light emitted from the light source” will also be referred to as “outgoing light” wherever appropriate hereinafter.
The above light source 211 uses a semiconductor laser which emits a laser beam. The composite optical element 212 is an integral combination of the three-beam generating diffraction grating 212a and beam-splitter diffraction grating 212b. The three-beam generating diffraction grating 212a splits, by diffraction, the light beam emitted from the light source 211 into three beams including a zero-order light beam and positive and negative first-order light beams to provide a tracking error signal by the so-called three-beam method. The beam-splitter diffraction grating 212b splits, by diffraction, return light from the optical disk 204 into a zero-order light beam and positive and negative first-order light beams and separates, from the outgoing light, the positive first-order light beam, for example, as return light to be directed to the photodetector assembly 216.
The photodetector assembly 216 includes a main-beam photodetector 221, shown in FIGS. 2A, 2B and 2C, that detects a zero-order light beam as a result of the splitting, by diffraction, of the return light by the three-beam generating diffraction grating 212a, and a pair of side-beam photodetectors (not shown) that detect positive and negative first-order light beams, respectively, resulted from the splitting, by diffraction, of the return light by the three-beam generating diffraction grating 212a. 
The optical system 201 uses the so-called astigmatism to provide a focusing error signal. On this account, a main-beam photodetector 221 included in the photodetector assembly 216 has a generally square light-detecting surface whose split pattern is quadrisected into four light-detecting areas a5, b5, c5 and d5 by a set of parting lines passing by the center of the light-detecting surface and perpendicular to each other as shown in FIGS. 2A, 2B and 2C. The light-detecting surface detects return light from the optical disk 204. The side-beam photodetectors (not shown) are disposed across the main-beam photodetector 221 oppositely to each other.
In the optical system 201, optical parts are disposed along the forward light path from the light source 211 to the optical disk 204 in such a manner that an image point being a conjugate point of the light-emitting point, as object point, of the light source 211 will be positioned on a recording layer 205 in the optical disk 204, as shown in FIG. 1. Also, in the optical system 201, the optical parts are disposed along the backward light path from the optical disk 204 to the photodetector assembly 216 in such a manner that the image point being a conjugate point of a point, as object point, on the recording layer 205 in the optical disk 204 will be positioned on the light-detecting surface of the main-beam photodetector 221 of the photodetector assembly 216.
Therefore, in the optical system 201, the light-emitting point of the light source 211 is also in a conjugate relation with the point on the light-detecting surface of the main-beam photodetector 221 of the photodetector assembly 216.
A focusing error signal is provided by the light-detecting areas a5, b5, c5 and d5 of the above-mentioned main-beam photodetector 221 as will be described below.
First, when the objective lens 215 is placed in an optimum position, namely, in a so-called just-in-focus position, in relation to the recording layer 205 in the optical disk 204, a beam spot defined on the light-detecting surface of the main-beam photodetector 221 will be circular as shown in FIG. 2B.
However, if the objective lens 215 is excessively close to the recording layer 205 in the optical disk 204, it will be off the just-in-focus position, and return light split by the beam-splitter diffraction grating 212b and passing by the composite optical element 212 will cause astigmatism that will cause the beam spot defined on the light-detecting surface of the main-beam photodetector 221 to have the form of an ellipse whose major axis extends over the light-detecting areas a5 and c5 as shown in FIG. 2A.
Further, if the objective lens 215 is excessively apart from the recording layer 205 in the optical disk 204, it will be off the just-in-focus position, and return light split by the beam-splitter diffraction grating 212b and passing by the composite optical element 212 will cause astigmatism that will cause the beam spot defined on the light-detecting surface of the main-beam photodetector 221 to have the form of an ellipse whose major axis extends over the light-detecting areas b5 and d5 as shown in FIG. 2C, namely, an ellipse whose major axis is inclined 90 deg. in relation to that of the beam spot shown in FIG. 2A.
Given that return-light detection outputs from the light-detecting areas a5, b5, c5 and d5 of the main-beam photodetector 221 are Sa5, Sb5, Sc5 and Sd5, respectively, a focusing error signal FE can be calculated using the following formula (1):FE=(Sa5+Sc5)−(Sb5+Sd5)  (1)
More specifically, in the main-beam photodetector 221, when the objective lens 215 is in the in-focus position, namely, in the so-called just-in-focus position, as shown in FIG. 2B, the focusing error signal FE given by the formula (1) will be zero.
Also in the main-beam photodetector 221, if the objective lens 215 is excessively close to the recording layer 205 in the optical disk 204, the focusing error signal FE will be positive. On the contrary, if the objective lens 215 is excessively apart from the recording layer 205 in the optical disk 204, the focusing error signal FE will be negative.
A tracking error signal TE is provided by calculating a difference between outputs from the side-beam photodetectors having detected positive and negative first-order light beams, respectively, from the three-beam generating diffraction grating 212a. 
In the optical pickup device using the optical system 201 configured as above, the objective lens 215 is moved to the in-focus position in relation to the recording layer 205 in the optical disk 204 according to the focusing error signal FE from the main-beam photodetector 221 of the photodetector assembly 216 and tracking error signal TE from the side-beam photodetectors, the outgoing light is focused on the recording layer 205 in the optical disk 204 and information is read from the optical disk 204.
Note here that generally, the light source 211, such as semiconductor laser, emits light whose wavelength depends upon the ambient temperature. Given that the ambient temperature is T, the wavelength of the laser beam emitted from the semiconductor laser at the ambient temperature T can be expressed approximately as given by the following formula (2):λT=λ0+c·ΔT  (2)where λT is a wavelength of light emitted at the ambient temperature T, λ0 is a wavelength at the normal temperature, ΔT is a temperature deviation from the normal temperature, and c is a temperature coefficient.
Also, in case the laser beam is incident at an angle θ upon the diffraction grating such as the aforementioned beam-splitter diffraction grating 212b in which it is diffracted at an angle θ′, the relation between the incident angle θ and diffraction angle θ′ can be expressed as given by the following formula (3):n′·sin θ′−n·sin θ=m·λ/d  (3)where λ is the wavelength of the laser beam, d is the grating constant of the diffraction grating, m is the grating order of the diffraction grating, n is the refractive index of incident-side medium of the diffraction grating and n′ is the refractive index of an outgoing-side medium of the diffraction grating.
In the aforementioned optical system 201, since the beam-splitter diffraction grating 212b of the composite optical element 212 diffracts return light with a refractive index of n=1 at an incident angle of θ=θ with respect to the main beam, the following formula (4) can be derived from the formula (3) on the assumption that the diffraction order m is +1:n′·sin θ′=λ/d  (4)
In case the ambient temperature around the optical system 201 varies, the following formula (5) can be provided by placing formula (2) in formula (4) with the diffraction angle at the ambient temperature T being taken as θ′T:n′·sin θ′T=(λ0+c·ΔT)/d  (5)
Further, the following formula (6) can be derived from the above formula (5) with the diffraction angle at the normal temperature given as θ′0:n′·sin θ′T=n′·sin θ′0+c·ΔT/d  (6)
Based on the above formula (6), the diffraction angle θ′T at the ambient temperature T can be expressed as given by the following formula (7):θ′T=θ′0+sin−1 ((c·ΔT)/(d·n′))  (7)
It is known from the above formula (7) that the diffraction angle θ′T of return light at the ambient temperature T depends upon the deviation ΔT from the normal temperature, that is, on the ambient temperature around the optical system 201.
The optical pickup device is produced at the normal temperature. Therefore, the position of the photodetector assembly 216 is adjusted on the assumption that return light is diffracted at an angle θ′0. However, if the ambient temperature varies after the position of the photodetector assembly 216 is adjusted, the return light will be diffracted at an angle that varies as given by the formula (7), and the center of a beam spot defined on the light-detecting surface of the main-beam photodetector 221 of the photodetector assembly 216 will be off a predetermined position as shown in FIG. 3.
In the aforementioned optical system 201 provided in the optical pickup device, when the photodetector assembly 216 provides a focusing error signal FE, if the center of a beam spot defined on the light-detecting surface of the mea-beam photodetector 221 is even slightly off that of the main-beam photodetector 221 in any direction, the output from the main-beam photodetector 221, which would be when the objective lens 215 is in the just-in-focus position, will not be zero. Consequently, the focusing error signal FE will be offset.
As above, in the optical pickup device, since the focus is controlled to obtain a focusing error signal FE equal to zero, the objective lens 215 cannot be driven to move precisely to any in-focus position.
In the optical pickup device including the optical block allowing light to pass through as above, astigmatism caused in the optical block will cause return light not to be appropriately collected to a desired position, so that the beam spot incident upon the light-detecting surface of the main-beam photodetector 221 of the photodetector assembly 216 will not have any appropriate generally circular form.
Also in this case, any focusing error signal FE cannot appropriately be provided in the optical pickup device.